Crescent Fracture (SI Joint Fracture-Dislocation, Day Classification)

The crescent fracture results from a lateral compression force applied to the pelvis. Posteriorly, this disrupts the sacroiliac joint and the injury propagates as a fracture through the posterior iliac wing, splitting off a crescent-shaped fragment of ilium that remains attached to the sacrum via the strong, intact posterior sacroiliac ligaments. Because those posterior ligaments stay attached to the crescent fragment, the injury is generally vertically stable but rotationally unstable. It is the posterior component of a Young-Burgess lateral-compression pelvic-ring injury and is usually accompanied by an anterior ring (pubic rami) injury.

The Day classification is useful because it directs the approach and reduction technique. The larger the crescent (Type I), the more the posterior stability resides in the iliac fragment, so fixing the iliac fracture restores the SI joint; the smaller the crescent (Type III), the more the injury is a joint dislocation, so iliosacral screw fixation across the SI joint is needed. Type II sits in between. (Some crescents do not fit neatly into the three types, and the scheme has been proposed for expansion.)

• Type I (large crescent): open reduction and internal fixation of the iliac fracture (anterior or posterior iliac approach; plates and/or lag screws) - this re-establishes SI-joint stability.
• Type III (small crescent): behaves like an SI dislocation - closed (or open) reduction and percutaneous ILIOSACRAL (sacroiliac) SCREW fixation across the SI joint.
• Type II (intermediate): a combination - reduce the joint and stabilise both the fracture and the SI joint as needed (often iliosacral screws +/- iliac fixation). Many crescent fractures are amenable to closed reduction and percutaneous iliosacral screw fixation, and in selected stable patterns non-operative management or external fixation has a role; the choice is individualised to the fracture type and the patient.